Bioinformatics Unit, National Institute on Aging, National Institutes of Health, Baltimore, Maryland, United States of America.
PLoS One. 2012;7(8):e42350. doi: 10.1371/journal.pone.0042350. Epub 2012 Aug 24.
Embryonic stem cells (ESCs) are pluripotent and have unlimited self-renewal capacity. Although pluripotency and differentiation have been examined extensively, the mechanisms responsible for self-renewal are poorly understood and are believed to involve an unusual cell cycle, epigenetic regulators and pluripotency-promoting transcription factors. Here we show that B-MYB, a cell cycle regulated phosphoprotein and transcription factor critical to the formation of inner cell mass, is central to the transcriptional and co-regulatory networks that sustain normal cell cycle progression and self-renewal properties of ESCs. Phenotypically, B-MYB is robustly expressed in ESCs and induced pluripotent stem cells (iPSCs), and it is present predominantly in a hypo-phosphorylated state. Knockdown of B-MYB results in functional cell cycle abnormalities that involve S, G2 and M phases, and reduced expression of critical cell cycle regulators like ccnb1 and plk1. By conducting gene expression profiling on control and B-MYB deficient cells, ChIP-chip experiments, and integrative computational analyses, we unraveled a highly complex B-MYB-mediated transcriptional network that guides ESC self-renewal. The network encompasses critical regulators of all cell cycle phases and epigenetic regulators, pluripotency transcription factors, and differentiation determinants. B-MYB along with E2F1 and c-MYC preferentially co-regulate cell cycle target genes. B-MYB also co-targets genes regulated by OCT4, SOX2 and NANOG that are significantly associated with stem cell differentiation, embryonic development, and epigenetic control. Moreover, loss of B-MYB leads to a breakdown of the transcriptional hierarchy present in ESCs. These results coupled with functional studies demonstrate that B-MYB not only controls and accelerates cell cycle progression in ESCs it contributes to fate decisions and maintenance of pluripotent stem cell identity.
胚胎干细胞（ESCs）具有多能性和无限的自我更新能力。尽管多能性和分化已经得到了广泛的研究，但负责自我更新的机制仍知之甚少，据信涉及异常的细胞周期、表观遗传调节剂和多能性促进转录因子。在这里，我们表明 B-MYB，一种细胞周期调节的磷酸化蛋白和转录因子，对内细胞团的形成至关重要，是维持正常细胞周期进程和 ESC 自我更新特性的转录和共调节网络的核心。表型上，B-MYB 在 ESCs 和诱导多能干细胞（iPSCs）中强烈表达，并主要处于低磷酸化状态。B-MYB 的敲低导致功能细胞周期异常，涉及 S、G2 和 M 期，以及关键细胞周期调节剂如 ccnb1 和 plk1 的表达降低。通过对对照和 B-MYB 缺陷细胞进行基因表达谱分析、ChIP-chip 实验和综合计算分析，我们揭示了一个高度复杂的 B-MYB 介导的转录网络，该网络指导 ESC 的自我更新。该网络包含所有细胞周期阶段和表观遗传调节剂的关键调节剂、多能性转录因子和分化决定因素。B-MYB 与 E2F1 和 c-MYC 一起优先共调节细胞周期靶基因。B-MYB 还共同靶向 OCT4、SOX2 和 NANOG 调节的基因，这些基因与干细胞分化、胚胎发育和表观遗传控制密切相关。此外，B-MYB 的缺失导致 ESCs 中存在的转录层次结构的崩溃。这些结果加上功能研究表明，B-MYB 不仅控制和加速 ESCs 中的细胞周期进程，还促进命运决定和多能干细胞特性的维持。
